The present invention relates, generally, to electric heating devices. More particularly, the invention relates to overheat protection for an electric space heater. The invention has particular utility in wall-mounted electric room heaters.
The basic operation of all electric room heaters is the same. A fan takes in room air in an inlet area and moves it across electrical heating elements, which warm the air. The heated air is then discharged from a discharge area back into the room. Typically there is no variable power condition for the heating elements; they are either off or on. When the heating elements are on, a minimum amount of heat must be transferred from the heating elements to the air flowing across them to keep the temperature of the heating elements below a desired level. If there is too little heat transfer, the heating elements can overheat and potentially burn out or cause a fire.
Heat transfer from the heating elements to the air flowing over them depends on the initial temperature of the air flowing across the heating elements and the amount of the airflow. If the airflow amount is reduced, the air will be in contact with the heating elements longer than desired, the outlet temperature will be the significantly higher than desired, and the temperature of the heating elements will also be significantly higher than desired. Likewise, if the initial inlet air temperature is significantly higher than it should be, even with the proper airflow amount, the outlet temperature and the temperature of the heating elements can be significantly higher than desired.
The state of the art includes various devices and methods for detecting an abnormal heating condition, particularly overheating, in an electric heater used to heat room air. Many heaters have a temperature limit switch located near the heating element or output area to detect an over-temperature condition.
U.S. Pat. Nos. 4,755,653 and 5,007,103 disclose heaters with circuits that interrupt power to the heating elements when a sensor detects an internal temperature greater than a predetermined maximum. However, those patents do not disclose a particular location for sensing the internal temperature.
In U.S. Pat. No. 5,083,011 to Munroe, two heater control signals are produced. One signal is a minimum pressure differential between static air pressure outside a hollow housing and the air velocity pressure within the housing. The other signal is a temperature of a portion of the housing lower than a predetermined maximum. The heater is actuated only when both of those signals are present.
U.S. Pat. No. 5,245,691 to Wu discloses an electric circuit for a heater which interrupts power to the heater when either an internal temperature exceeds a limit or the heater is tilted.
A heater incorporating more safety devices is disclosed in U.S. Pat. No. 5,805,767 to Jouas. The heater element is deenergized if any of the following occur: the internal temperature of the heater exceeds a set maximum, the rate at which the internal temperature is increasing exceeds a given rate, the internal temperature exceeds the ambient air temperature by a given amount, or when motion is detected near the front of the heater. The features disclosed in Jounas are more proactive than the purely reactive features of the other patents in that they can shut off the heating element before it has become too hot, but the features are relatively complex and expensive.
The present invention provides an improved electrical heater which overcomes the limitations and shortcomings of the prior art.